The present invention relates to a system for non-destructive testing of metallic workpieces, and, more particularly, for the testing of elongated cylindrical members such as tubes or pipes, to detect both longitudinal and transverse defects.
Procedures are known in the prior art for the magnetic non-destructive testing of metallic workpieces. These systems utilize one of two methods of magnetic testing. The first is the so-called eddy current method, in which a high-frequency alternating magnetic field is used to produce eddy currents in the workpiece surface layer and the characteristics of the eddy currents are related to the physical condition of the workpiece. A sensing element located near the workpiece produces an electric signal responsive to the fields reradiated by these eddy currents.
The second approach is the so-called stray field or leakage flux method, whereby a magnetic field is generated within the workpiece and the field in the adjacent air is monitored. Defects which cut across the lines of flux of the magnetic field will generate so-called stray fields adjacent the surface of the workpiece, and magnetic sensing elements detecting the stray fields produce an electric signal indicating the presence of the defects.
The eddy current method may be used for the testing of any metallic workpiece; however, since the eddy currents only penetrate a very short distance below the surface of the workpiece, it is not useful for testing the entire thickness of a relatively thick workpiece. The stray field method must be used on ferromagnetic workpieces; however, it will detect the presence of a defect located anywhere throughout the entire thickness of the workpiece.
Non-destructive testing systems using both of these methods are also known in the prior art. However, there are many shortcomings associated with the known systems which are overcome by the present invention, one of the more important of these shortcomings being the inability to detect both transversely and longitudinally extending defects. Secondly, those parts of the systems relating to the stray field method frequently utilize elongated sensing coils positioned adjacent the surface of the workpiece, which produce signals that are not proportional to the depth of a defect, whereas standards of the testing art usually require the rejection of a workpiece having a defect with a depth which exceeds certain limits. Moreover, the magnitude of a signal produced by such a search coil depends not only on the size of the defect, but also on the relative speed at which the coil moves over the surface and the direction of the defect. Still further, when testing elongated cylindrical workpieces such as pipes, a separate coil must be provided for each pipe diameter, since the curvature of the coils must match the curvations of the pipes exactly. Also, known stray field systems have been unable to satisfactorily resolve ID and OD defect signals.